Since the traffic geographic information is increasingly complex, and if public transit vehicle is considered, it may not satisfy everyone specific requirements because it may only arrive at any specific locations, therefore, a navigation device that directs drivers to any specific place in personal need is gradually popular. Generally, a navigation device plans optimized routes according to map information and the specific requirement such as the current location and a destination, and provides voice guidance informing the user how to follow an optimized route. The information of an optimized route consists of a series of voice guidance such as a distance to a cross road or turn-by-turn navigation instructions as well as road information such as the name of the road, the section of the road, and the type of the road. It thus allows users to drive from the current location to a destination under the users' requirement in a way of most minimal cost. However, in negligence or an emergency, users may not follow the guidance of the navigation device so as to deviate from the optimized route. Although the navigation device then still can guide the users another new optimized route based on the current location and map information, the extra cost needed remains serious. In addition to the time-cost, the cost further comprises the oil-cost and extra fee, such as the toll to pass a bridge.
Further, the deviation from the optimized route on different locations causes different results. For example, if a user ignores turning voice guidance A and thus misses an interchange in highway, the user at least needs to drive to the next interchange. Whereas a user ignores a turning voice guidance B in a small town, then the user only needs to make a U-turn or turns in the next intersection according to the new optimized route. The inconvenience caused by the two scenarios of user differs. Therefore, the importance of voice guidance within the optimized route thus makes different. Prior art treats the importance of all voice guidance as the same, so that users must judge the importance of each voice guidance by themselves. However, if users are not familiar with the local map, they can not determine the importance of each voice guidance in time. As a result, users are to neglect the voice guidance with higher importance to deviate substantially from the optimized route. Moreover, a user is difficult to consider the priority of the voice guidance when driving.